Recently i had to migrate a LVM based domU from machine 1 to machine 2 with only ssh port being available between the two hosts. Of course dd comes to the rescue for that but i admit that having some informations about transfer rate would be interesting. And then i remembered a Sébastien's blog post talking about about a nice tool called PV. Of course PV has nothing to do with PV as in Physical Volume for LVM but it's a 'pipe viewer' . A pv rpm is available in the RPMForge repo. Example (assuming that you've already created a domU2migrate lv on the target system) :

[root@machine2 \~]\# ssh machine1 "dd if=/dev/VolGroup00/domU2migrate"|pv -s 8G -petr|dd of=/dev/xen02vg/domU2migrate  
0:00:30 [11.2MB/s] [====\>      ]  4% ETA :10:13

I hope you'll find that useful if you never heard of such tool ..